Location from Percy Jackson by Rick Riordan
The pit beneath the Underworld — where defeated Titans and monsters reform, and where Percy and Annabeth survived the worst journey in the series.
Tartarus is where monsters go when demigods kill them — they dissolve, reform in Tartarus, and eventually crawl back to the mortal world. Percy and Annabeth fell in during House of Hades and had to cross it on foot to reach the Doors of Death. It's the most harrowing arc in the series — two teenagers navigating literal Hell, drinking from the River Phlegethon (liquid fire) to survive, and relying on Bob the janitor-Titan to guide them.
An endless dark pit with a toxic atmosphere, rivers of fire and sorrow, and a landscape made of the body of Tartarus himself (he's both a place and a primordial being). The air is poison to mortals, the ground is flesh, and every monster that has ever been killed is reforming somewhere in its depths. It smells like sulfur and despair.
Also known as: Tartarus, the Pit